The present invention relates to the production of multiple-layer film or sheeting, and more especially, to a process and apparatus for the extrusion of multiple-layer sheeting having a layer or layers of polymeric material having desirable surface characteristics. More especially, the present invention pertains to the co-extrusion of multiple-layer sheeting, with each layer exhibiting uniform layer thickness across the width thereof, by an apparatus which has an improved laydown means and which is easily adjustable to produce sheeting products having desired thicknesses.
Co-extrusion processes for the production of multi-layer sheeting are, of course, well known in the art. However, prior processes do not guarantee, and often do not produce, a co-extruded layer of uniform thickness on the main extruded material. This problem is especially acute when the co-extruded layer comprises a polymeric material having a comparatively low viscosity. Such a material tends to migrate from areas of higher extruder die pressure, to areas of lower extruder die pressure. This results in a sheeting product having varying layer thicknesses across the width of the sheet.
On the other hand, when the polymeric material extruded is one of high viscosity, the polymer will tend to remain at its central entry point on the sheet and will not migrate. This results in a sheeting product having a layer of decreasing thickness away from the center.
Furthermore, in known processes and apparatus, it was not readily possible to adjust the relative thickness of the various layers, in response to different processing characteristics and/or different requirements for the final products. Crude control of pressures and flow rates was utilized in an attempt to monitor the quantity of co-extrudant being applied to the sheeting. For example, a plurality of valve plates were machined, each providing a different flow rate of co-extruded polymer. These were manually exchanged according to process requirements. This type of control did not, in many cases, provide the level of control necessary in many processes and products.
The present invention is directed to an improved laydown means for the application of multiple-layers of polymeric material to an initial polymeric base layer, the different polymeric materials in most cases exhibiting different processing characteristics. The laydown device permits easy adjustment of overall layer thickness.
The present invention is also concerned with an improved laydown means and process for use in the production of multiple-layer sheeting of materials, particularly those which have different processing characteristics, resulting in a uniformity of layer thickness across the entire width of the extruded sheeting.